The invention relates to semiconductor manufacturing and more specifically to a method and apparatus for controlling the delivery of slurry through a polishing pad on a chemical mechanical polishing (CMP) tool.
A flat disk or xe2x80x9cwaferxe2x80x9d of single crystal silicon is the basic substrate material in the semiconductor industry for the manufacture of integrated circuits. Semiconductor wafers are typically created by growing an elongated cylinder or boule of single crystal silicon and then slicing individual wafers from the cylinder. The slicing causes both faces of the wafer to be extremely rough. The front face of the wafer on which integrated circuitry is to be constructed must be extremely flat in order to facilitate reliable semiconductor junctions with subsequent layers of material applied to the wafer. Also, the material layers (deposited thin film layers usually made of metals for conductors or oxides for insulators) applied to the wafer while building interconnects for the integrated circuitry must also be made a uniform thickness.
Planarization is the process of removing projections and other imperfections to create a flat planar surface, both locally and globally, and/or the removal of material to create a uniform thickness for a deposited thin film layer on a wafer. Semiconductor wafers are planarized or polished to achieve a smooth, flat finish before performing process steps that create the integrated circuitry or interconnects on the wafer. A considerable amount of effort in the manufacturing of modern complex, high density multilevel interconnects is devoted to the planarization of the individual layers of the interconnect structure. Nonplanar surfaces create poor optical resolution of subsequent photolithography processing steps. Poor optical resolution prohibits the printing of high-density lines. Another problem with nonplanar surface topography is the step coverage of subsequent metalization layers. If a step height is too large there is a serious danger that open circuits will be created. Planar interconnect surface layers are required in the fabrication of modem high-density integrated circuits. To this end, chemical-mechanical polishing (CMP) tools have been developed to provide controlled planarization of both structured and unstructured wafers.
CMP consists of a chemical process and a mechanical process acting together, for example, to reduce height variations across a dielectric region, clear metal deposits in damascene processes or remove excess oxide in shallow trench isolation fabrication. The chemical-mechanical process is achieved with a liquid medium containing chemicals and possibly abrasive particles (commonly referred to as slurry) that react with the front surface of the wafer while it is mechanically stressed during the planarization process.
In a conventional CMP tool for planarizing a wafer, a wafer is secured in a carrier connected to a shaft. Pressure is exerted on the back surface of the wafer by the carrier in order to press the front surface of the wafer against the polishing pad in the presence of slurry. The wafer and/or polishing pad are then moved in relation to each other via motor(s) connected to the shaft and/or platen in order to remove material in a planar manner from the front surface of the wafer. Various combinations of motions are known for moving the wafer and polishing pad in relation to each other. For example, the wafer is commonly rotated or held stationary and the polishing pad is moved in either a linear, rotational or orbital manner.
A common problem in CMP is for the wafer to polish in a nonplanar manner. The wafer typically has a xe2x80x9cbull""s-eyexe2x80x9d pattern with the center of the wafer being polishing either faster or slower than the circumference. The polishing rate tends to be uniform within concentric bands, but not across the entire surface of the wafer. Numerous attempts have been made to remedy this problem with only partial success. This problem has recently worsened as some of the slurries used to planarize wafers with copper thin films result in nonuniform material removal with limited process control.
One attempted solution for the situation where the center of the wafer is being polished too slowly is to move the edge of the wafer over the edge of the polishing pad. This will slow the removal rate of material at the edge of the wafer to approximate the removal rate at the center of the wafer. This solution is relatively inexpensive, but has several problems. One problem is that this solution is not able to compensate for the center fast situation. In addition, front-reference carriers (those supporting the wafer by air or a membrane) tend to break or lose control of the wafer when the wafer is placed over the edge of the polishing pad. Another problem is that this approach has minimal flexibility in fine tuning the removal rate over the entire surface of the wafer.
Another attempted solution is to use a multizone carrier. Multizone carriers typically have a central zone and one or more concentric zones for altering the polishing rate for corresponding concentric zones on the wafer. Each of the zones in the carrier may be configured to apply an individually controllable pressure on the back surface of the wafer. In this way, concentric bands that are polishing too quickly or too slowly on the front surface of the wafer may receive a correcting lower or higher pressure on the back surface of the wafer by the multizone carrier. This approach adds more flexibility to the process, but also adds a great deal of expense and complexity to the process.
What is needed is a method and apparatus for uniformly planarizing a wafer that avoids the problems of the prior art. The solution needs to provide flexibility to the planarization process to correct for nonuniform polishing, while remaining simple and cost-effective.
The present invention improves the planarization process of a wafer during a chemical mechanical polishing process while avoiding the problems of the prior art. An object of the invention is to provide a method and apparatus that may be used to alter the removal rate of material from the front surface of the wafer to compensate for nonuniform polishing. Another object of the invention is to provide an apparatus and method for controlling the distribution of slurry to the top surface of the polishing pad.
The apparatus includes a platen having a bottom surface, a top surface and a plurality of circular walls (having different radial dimensions) extending from the bottom surface to the top surface. The bottom surface, top surface and circular walls define a plurality of concentric plenums within the platen. The bottom surface, top surface and circular walls may be manufactured as one, two or three separate pieces and may be made from the same or different materials. The bottom surface has a plurality of holes that a plurality of fluid delivery systems delivers fluids through to each plenum. The holes may be along a single diameter of the bottom surface or may be spaced apart to simplify the manufacturing process. The top surface also has holes for allowing the fluids, once inside the plenums, to travel through to reach the polishing pad. The holes in the top surface may be strategically placed to enhance the distribution of slurry on the top surface of the polishing pad.
The circular walls may be an integral part of the bottom or top surface of the platen or may be a separate component. The circular walls preferably provide fluid tight separators between the plenums and may be an o-ring. If o-rings are used, grooves are preferably created in the bottom and/or top surface of the platen to accept the o-rings.
A polishing pad may be supported on the top surface of the platen. The polishing pad may include a plurality of holes that align with the holes in the top surface of the platen to facilitate fluid flowing from the plenums through the top surface of the platen and through the polishing pad.
A metrology instrument may also be used, preferably having at least one probe near each plenum, for measuring a front surface of the wafer. Fewer probes may be used to simplify and reduce the cost of the metrology instrument or additional probes may be used to enhance the capabilities of the metrology instrument. The metrology instrument may be, for example, a multiprobe end-point detection system. Data regarding the measurements may be used by a computer to determine if corrections to the planarization process may advantageously be made.
Each fluid delivery system may include a tank, pump, valve and computer. The tank stores the slurry and may be used with other fluid delivery systems and/or other CMP tools. The pump communicates the fluids from the tank to the plenums and includes traditional pumps or gravity delivery systems. Valves may be used to regulate the flow of fluids from the tank to the plenums. A computer (network, stand alone, etc.) may be used to automate the entire CMP tool and control the functions of the pump and/or valves. The fluids used may be mixed in the tank. This will simplify the number of fluid delivery systems needed. However, the preferred method is to have a separate fluid delivery system for each fluid and to mix the fluids as close to the point of use as possible. This keeps the fluids from undesirably interacting with each other prior to use.
The preferred motion for the polishing pad is an orbital motion created by an orbital motion generator connected to the bottom surface of the platen. However, one skilled in the art will recognize that the invention may be used with other motion platforms such as a rotational or linear with only minor modifications to the embodiments specifically illustrated. A small radius for an orbital motion of the polishing pad may be used to effectively planarize a wafer. The small radius for the orbital motion allows the center, middle and periphery of the polishing pad to stay near the corresponding center, middle and periphery of the wafer during the planarization process. This fact simplifies the process of determining the effects of different flow rates and composition of fluids distributions across the surface of the polishing pad.
In practice, a desired flow rate and composition of fluids may be communicated from the tanks to the top surface of the polishing pad via the fluid delivery systems as previously discussed. By utilizing a plurality of fluid delivery systems in fluid communication with different concentric plenums, a different flow rate and/or composition of fluids may be delivered to different concentric portions on the top surface of the polishing pad.
The polishing pad is moved in relation to the front surface of the wafer, preferably orbited, as the wafer is pressed against the polishing pad to planarize the wafer. The planarization process may be monitored and the wafer measured by a metrology instrument that preferably has at least one probe that corresponds to the location of each plenum. Fewer probes than the number of plenums may be used to simplify and lower the cost of the metrology instrument. A change in the flow rate and/or composition of fluids from one or more of the plurality of plenums to the top surface of the polishing pad during the planarization process may be made based on the measurements. In addition, the measurements from the wafer during the planarization process may be used to determine a new desired flow rate and composition for subsequent wafers.